Lens, lighting covers, personal computer parts and display parts have mostly employed a methacrylic resin having advantage of excellent optical features.
Recently, liquid crystal displays and liquid crystal televisions have grown in size as they are widely used. To prevent warp caused by moisture absorption, a resin such as a methyl methacrylate/styrene copolymer, whose moisture absorption is lower than that of a methacrylic resin, has come to be often used as the material for a light-diffusion plate of a backlight. Indeed, the degree of warp caused by moisture absorption is reduced by reducing moisture adsorption but the reduction level is not sufficient. Then, polystyrene exhibiting much lower moisture absorption has come to be used.
As a method for imparting light diffusibility to a transparent resin, an approach of blending fine particles having different refractive indexes with the transparent resin has been disclosed from long ago. For example, mention may be made of a technique for blending 1-10 μm fine particles (Patent Document 1); a technique for blending 10-50 μm fine particles (Patent Document 2); a technique for blending 1-6 μm silicone resin fine particles in combination with 1-7 μm inorganic fine powder (Patent Document 3); a technique for blending crosslinked-resin fine particles of less than 5 μm in combination with crosslinked-resin fine particles of 5-10 μm (Patent Document 4); and a technique for blending a 1-20 μm light diffusing agent (Patent Document 5). However, these fine particles, regardless of whether they are organic or nonorganic, have a problem of low affinity with the resin, decreasing impact resistance.
To improve impact resistance, a technique for adding an acrylic multi-layer polymer is disclosed (Patent Documents 6 to 8). However, since the multi-layer polymer particles, which are produced by emulsion polymerization generally have as small a diameter as less than 0.5 μm, the yellow index of the transmitted light is high. For this reason, the multi-layer polymer particles are not preferable.
As the styrene-based resin having impact resistance, a rubber-modified styrene-based resin (impact resistant polystyrene) is known. However, conventional rubber-modified styrene-based resin has a problem in that the yellow index of transmitted light is high. In addition, there is a problem of low diffuse-transmission of light. Patent Document 9 discloses a rubber-modified styrene-based resin having excellent light transmissivity and diffusibility, which is obtained by dispersing particles such that the diameter and the accumulation volume ratio of the particles are allowed to fall within predetermined ranges. By virtue of this technique, light diffusion transmissivity is improved; however, the yellow index of the transmitted light is high. Improvement of the yellow index has been desired.
Patent Document 1: JP-B-60-21662
Patent Document 2: JP-A-60-139758
Patent Document 3: Japanese Patent No. 2512544
Patent Document 4: JP-A-11-60966
Patent Document 5: JP-A-2004-50607
Patent Document 6: JP-A-08-198976
Patent Document 7: JP-A-2000-296581
Patent Document 8: JP-A-2004-90626
Patent Document 9: JP-A-2003-2937